WDM communication system utilizing WDM optical sources with stabilized wavelengths and light intensity and method for stabilization thereof

Abstract

A multichannel WDM transmission system incorporates a plurality of WDM optical sources with stabilized wavelengths and light intensity. Efficient stabilization of these characteristics is achieved by modulation of WDM sources by distinguishing low frequency electrical signals in a range between 1 and 4 kHz and modulation depth in a range between 1% and 5% that are used as WDM source identifiers. After the modulated outputs of the WDM sources are multiplexed and filtered, a Fourier transform of total light intensity may be obtained. Digital feedbacks provide stabilization of both the wavelength and light intensity of each WDM optical source.

What is claimed is:  
     
       1. A WDM communication system for propagating a plurality of optical signals produced by a corresponding plurality of WDM optical sources via an optical fiber comprising: 
       a transmission system for generating and transmitting said optical signals, each said optical signal modulated by a distinguishing electrical signal of low frequency and small modulation depth, and mixing each said modulated optical signal into a WDM optical signal, each said low frequency signal being a distinguishing identifier for each said WDM optical source;  
       a detection system coupled to said transmission system for detecting a portion of said WDM optical signal and obtaining first and second electrical signals carrying information on light intensity and wavelength respectively for each WDM optical source of said plurality of WDM optical sources; and  
       a control system coupled between said transmission system and said detection system for analyzing said first and second electrical signals by Fourier transform and obtaining information on wavelength and light intensity for each said WDM optical source utilizing said distinguished identifier, and adjusting said wavelength and light intensity.  
     
     
       2. The multichannel WDM communication system of  claim 1 , wherein said transmission system comprises: 
       a plurality of data modulators connected respectively to said plurality of WDM optical sources for modulating each said WDM optical source by a respective electrical signal of low frequency and small modulation depth;  
       a low frequency generator coupled to said plurality of data modulators for generating a plurality of said distinguishing low frequency and small modulation depth electrical signals;  
       a variable optical attenuator unit for attenuating intensity of output signals of said data modulators;  
       a WDM multiplexer for combining output signals of said variable optical attenuators into said WDM optical signal; and  
       a sample signal unit for diverting a sample signal, said sample signal derived from said WDM optical signal, said sample signal being divided into first and second sample signals for transmitting via respective first and second transmission paths.  
     
     
       3. The multichannel WDM communication system of  claim 2 , wherein said plurality of distinguishing low frequency electrical signals have frequencies (f 1 , f 2 , . . . f n ) in a range of about 1 kHz to 4 kHz and providing a depth of modulation of said optical signals in a range of about 1% to 5%, wherein n is the number of WDM communication channels. 
     
     
       4. The multichannel WDM communication system of  claim 3 , wherein said sample signal unit further comprises: 
       an external tap coupler placed within said optical fiber for diverting about 1% of said WDM optical signal, and  
       a splitter for dividing said sample signal into said first and second sample signals for directing them into said first and second transmission paths.  
     
     
       5. The multichannel WDM communication system of  claim 4 , wherein said detection system comprises: 
       a first detector placed within said first transmission path for detecting and converting said first sample signal into a first electrical signal; and  
       a wavelength locker and a second detector that are placed within said second transmission path for providing wavelength selectivity of said second sample signal and converting it into a second electrical signal.  
     
     
       6. The multichannel WDM transmission system of  claim 5 , wherein said control system comprises: 
       an analyzer coupled to outputs of said first and second detectors for transforming said first and second electrical signals into Fourier components corresponding respectively to intensity and wavelengths of said WDM optical sources; and  
       feedback connectors coupled between said analyzer and said WDM optical sources and said analyzer and said variable optical attenuators for providing digital feedback to wavelength and light intensity of each said WDM source respectively.  
     
     
       7. The multichannel WDM communication system of  claim 1 , wherein said transmission system comprises: 
       a plurality of LiNbO 3  modulators connected respectively to said plurality of WDM optical sources for modulating each said WDM optical source by a respective electrical signal of low frequency and small modulation depth;  
       a bias modulator control unit for generating a plurality of said distinguishing low frequency and small modulation depth electrical signals;  
       a plurality of variable optical attenuators connected to said plurality of LiNbO 3  modulators respectively for attenuating the intensity of output signals of said LiNbO 3  modulators;  
       a WDM multiplexer for combining output signals of said variable optical attenuators into said WDM optical signal; and  
       a sample signal unit for diverting a sample signal, said sample signal derived from said WDM optical signal, said sample signal being divided into first and second sample signals for transmitting via respective first and second transmission paths.  
     
     
       8. The multichannel WDM communication system of  claim 7 , wherein each said LiNbO 3  modulator comprises a high frequency input for transmitting said optical signal of said WDM optical source and a low frequency input for applying said distinguishing electrical signal of low frequency and small modulation depth. 
     
     
       9. The multichannel WDM communication system of  claim 8 , wherein said plurality of distinguishing low frequency electrical signals have frequencies (f 1 , f 2 , . . . f n ) in a range of about 1 kHz to 4 kHz that provide a depth of modulation of said optical signals in a range of about 1% to 5%, wherein n is a number of WDM communication channels. 
     
     
       10. The multichannel WDM communication system of  claim 9 , wherein each of said variable optical attenuators equalizes the intensity of each said optical signal generated by each said WDM optical source. 
     
     
       11. The multichannel WDM communication system of  claim 10 , wherein said sample signal unit further comprises: 
       an external tap coupler placed within said optical fiber for diverting about 1% of said WDM optical signal, and  
       a splitter for dividing said sample signal into said first and second sample signals for directing them into said first and second transmission paths.  
     
     
       12. The multichannel WDM communication system of  claim 11 , wherein said detection system comprises: 
       a first detector placed within said first transmission path for detecting and converting said first sample signal into a first electrical signal; and  
       a wavelength locker and a second detector that are placed within said second transmission path for providing wavelength selectivity of said second sample signal and converting it into a second electrical signal.  
     
     
       13. The multichannel WDM communication system of  claim 12 , wherein said control system comprises: 
       an analyzer coupled to outputs of said first and second detectors for transforming said first and second electrical signals into Fourier components corresponding respectively to intensity and wavelengths of said WDM optical sources; and  
       feedback connectors coupled between said analyzer and said WDM optical sources, said analyzer and said variable optical attenuators providing digital feedback on wavelength and light intensity of each said WDM source respectively.  
     
     
       14. A multichannel WDM communication system for propagating optical signals via optical fiber, comprising: 
       a plurality of directly modulated diode lasers for generating optical signals within a respective plurality of communication channels;  
       a generator of low frequency electrical signals coupled to said plurality of directly modulated diode lasers for generating a plurality of distinguishing electrical signals of low frequency and small modulation depth applied to low frequency inputs of a corresponding plurality of said directly modulated diode lasers, each said distinguishing low frequency electrical signal being an identifier for a respective directly modulated diode laser;  
       a plurality of variable optical attenuators connected respectively to said plurality of directly modulated diode lasers for setting each said directly modulated diode laser to a predetermined optical power;  
       a WDM multiplexer for mixing outputs of said directly modulated diode lasers into a WDM optical signal;  
       a sample signal unit for diverting a sample portion of said WDM optical signal and dividing said sample portion about equally into first and second portions for transmitting them via first and second respective transmission paths; and  
       a control unit comprising:  
       a microprocessor for analyzing said first and second portions by converting them into first and second electrical signals respectively and for transforming said first and second electrical signals into Fourier components corresponding respectively to light intensity and wavelengths of said directly modulated diode lasers, the light intensity and wavelengths for each said directly modulated diode laser being obtained by utilizing said distinguishing identifier; and  
       feedback connectors coupled between said microprocessor and said directly modulated diode lasers, and said microprocessor and said variable optical attenuators for providing a respective digital feedback for controlling wavelength and light intensity of each said WDM source.  
     
     
       15. The multichannel WDM communication system of  claim 14 , wherein said plurality of distinguishing low frequency electrical signals have frequencies (f 1 , f 2 , . . . f n ) in a range of about 1 kHz to 4 kHz and provide said depth of modulation of said optical signals in a range of about 1% to 5%, where n is a number of WDM communication channels. 
     
     
       16. The multichannel WDM communication system of  claim 15 , wherein said sample signal unit further comprises an external tap coupler placed within said optical fiber for diverting about 1% of said WDM multiplexed signal into said transmission path, and a splitter for dividing said sample signal into said first and second portions. 
     
     
       17. The multichannel WDM communication system of  claim 16 , wherein said control unit further comprises; 
       a wavelength locker placed within said second transmission paths, said wavelength locker has selective elements for providing wavelength selectivity for said second portion of said sample; and  
       a detector connected to said wavelength locker for converting said second portion of said sample into said second electrical signal.  
     
     
       18. A method for stabilizing wavelengths and light intensity of WDM optical sources of a multichannel WDM communication system, comprising the steps of: 
       modulating said WDM optical sources by distinguishing electrical signals of low frequency and small modulation depth for obtaining a unique optical output from each said WDM optical source;  
       setting a predetermined optical power for WDM optical sources by attenuating each output of said WDM optical sources by variable optical attenuators;  
       combining said modulated optical outputs by a WDM multiplexer for obtaining a WDM optical signal;  
       decoupling a sample signal, said sample signal froming a small portion of said WDM optical signal;  
       diverting said sample signal and transmitting two approximately equal portions of said sample signal via two respective transmission paths;  
       detecting one portion of said sample signal and converting thereof to a first electrical signal;  
       providing wavelength selectivity for another portion of said sample signal by a wavelength locker and detecting said another portion of said sample signal for converting thereof into a second electrical signal;  
       analyzing said first and second electrical signals by applying Fourier transform for obtaining a spectrum of respective light intensities and wavelengths of said WDM optical sources corresponding to the Fourier components; and  
       identifying light intensity and wavelength of each said WDM optical source utilizing said unique optical output; and  
       providing digital feedback to said variable optical attenuators and WDM optical sources for stabilization of intensities and wavelengths thereof.  
     
     
       19. The method for stabilizing wavelengths and light intensity of WDM optical sources of  claim 18 , further comprising the step of amplifying of said WDM optical signal by an optical amplifier. 
     
     
       20. The method for stabilizing wavelengths and intensity of WDM optical sources of  claim 19 , wherein the step of modulating said WDM optical sources further comprises modulating said WDM optical sources by electrical signals having frequencies (f 1 , f 2 , . . . f n ) in a range of about 1 kHz to 4 kHz, and depth of modulations in a range of about 1% to 5%. 
     
     
       21. The method for stabilizing light intensity and wavelengths of WDM optical sources of  claim 20 , wherein said modulation frequencies of electrical signals exceed the inverse response time of said optical amplifier. 
     
     
       22. The method for stabilizing wavelengths and light intensity of WDM optical sources of  claim 21 , wherein said modulating frequencies are lower than the frequency components of data modulation. 
     
     
       23. The method for stabilizing wavelengths and light intensity of WDM optical sources of  claim 22 , wherein the step of analyzing said second electrical signal further comprises the step of obtaining the amplitude of the Fourier component given by 
       
         
           | F ( f   i )|=η< P   i0   >m/ 2;  
         
       
       where P i0  is the average power of an individual WDM optical source, f i  and m are respectively the modulating frequency and the depth of modulation of said individual WDM optical source. 
     
     
       24. The method for stabilizing wavelengths and light intensity of WDM optical sources of  claim 23 , wherein the step of analyzing said first electrical signal further comprises the step of obtaining the spectrum of said first electrical signal having said frequencies (f 1 , f 2 , . . . f n ).